Scanning electron beam computed tomography ("CT") systems are described generally in U.S. Pat. No. 4,352,021 to Boyd, et al. (Sep. 28, 1982), and U.S. Pat. No. 4,521,900 (Jun. 4, 1985), U.S. Pat. No. 4,521,901 (Jun. 4, 1985), U.S. Pat. No. 4,625,150 (Nov. 25, 1986), U.S. Pat. No. 4,644,168 (Feb. 17, 1987), U.S. Pat. No. 5,193,105 (Mar. 9, 1993), and U.S. Pat. No. 5,289,519 (Feb. 22, 1994), all to Rand, et. al. Applicants refer to and incorporate herein by reference each above listed patent to Rand, et al.
FIGS. 1 and 2 depict a generalized scanning electron beam computed tomographic X-ray system 8, such as described in the above-referenced Rand et al. patents. Referring to FIG. 2, as used herein, the terms "upstream" and "downstream" refer to relative position of elements or components, in which "downstream" elements are located to the right of "upstream" elements, the most "upstream" element being electron gun 32. Thus, electron gun 32 is "upstream" from beam optical assembly 38 (which of course is "downstream" from electron gun 32), and beam optical assembly 38 is "upstream" from target 14 (which is "downstream" from electron gun 32, and from beam optical assembly 38. System 8 includes a vacuum chamber housing 10 in which an electron beam 12 is generated at the cathode of an electron gun 32 located in upstream region 34, in response to perhaps -130 kV high voltage. This potential accelerates the electron beam downstream along the chamber axis 28. Further downstream, beam optical assembly 38 causes the electron beam to scan at least one circular X-ray emitting target 14, located within a front lower portion 16 of housing 10. Z-axis 28 preferably is coaxial with electron beam 12 upstream from the beam optical assembly 38 and is the longitudinal axis of chamber 10, and the axis of symmetry for beam optics assembly 38 and electrode assembly 44.
Beam optical assembly 38 is sometimes referred to as a magnetic and deflecting lens system. Assembly 38 includes a magnetic solenoid system comprising a magnetic solenoid and trim solenoid coils (collectively 39), quadrupole and deflection coils (collectively 42), and an electrode assembly 44. Electrode assembly 44 may include a rotatable transverse field ion clearing electrode ("RICE"), a positive ion electrode 48 ("PIE"), and ion clearing electrodes ("ICEs"). Beam assembly 44 electrodes are mounted within housing 10 between electron gun 32 and coils 39 and 42 such that the electron beam 12 passes axially therethrough about axis 28.
As the electron beam passes through the vacuum chamber, it ionizes residual or introduced gas (e.g., nitrogen at 10.sup.-6 Torr) therein, producing positive ions. The positive ions are useful in the downstream chamber region where space-charge neutralization and beam self-focusing are desired. But in the upstream region, unless removed by an external electrostatic field the positive ions would be trapped in the negative electron beam, and the space-charge needed for desired beam self-expansion would be undesirably neutralized.
As described in U.S. Pat. Nos. 4,625,150, 5,193,105, and 5,289,519, positive ions may be removed from the beam with a device that creates transverse electric fields in the region between the electron gun and the PIE. One form of this device is a rotatable ion clearing electrode assembly, referred to as a "RICE" unit.
RICE element 44 and the ICE elements remove positive ions while maintaining a uniform electric field. These elements are disclosed in U.S. Pat. No. 4,625,150 to Rand, et al. As noted in U.S. Pat. No. 5,386,445 to Rand, various components of the ICE and RICE elements may in fact be dispensed with.
As disclosed in U.S. Pat. No. 5,193,105, 5,289,419, and 5,386,445, PIE 48 is a planar washer through whose center opening the electron beam passes. The PIE is coupled to a large positive potential (e.g., +2.5 kV) to produce an axial field that blocks positive ions from migrating upstream. PIE 48 sharply defines the interface between upstream region 34 (where ions are removed) and downstream region 36 (where ions accumulate and neutralize the beam).
Whereas electrode assembly 44 controls positive ions in the upstream region, coils 39 and 42 contribute a focusing effect to help shape the final beam spot as it scans one of the targets 14. The final beam spot at target 14 should be elliptically shaped.
Target 14 emits a moving fan-like beam of X-rays 18 when scanned by focused electron beam 12. X-rays 18 then pass through a region of a subject 20 (e.g., a patient or other object) and register upon a detector array 22 located diametrically opposite. The detector array 22 and target(s) 14 are coaxial with and define a plane orthogonal to the system axis of symmetry 28. The detector array outputs data to a computer system (indicated by arrows 24 in FIG. 1). The computer system processes and records the data to produce an image of a slice of the subject on a video monitor 26. The computer system also controls the system 8 and the electron beam production therein.
Image resolution is maximized and target heating is minimized by maintaining an elliptical electron beam profile at the target, with the major axis normal to the sweep direction of the beam. In the X-Z azimuthal plane (containing the sweep direction) the waist of the beam must be located at the target. However, preferably the beam waist in the Y-Z radial plane is located upstream of the target to prevent target damage in the event of a pressure burst in the scanner system's vacuum system. The on-target beam dimension in the radial plane is a design specification that must be kept constant. The on-target beam dimension in the azimuthal plane is determined by the beam emittance, and depends upon the design of the electron gun.
As noted, the electron beam scanning system deflects the electron beam off the central Z-axis to the target ring using a pair of X and Y orthogonal dipole deflection coils. By varying the current to the X and Y coils, the beam position is swept azimuthally around the target ring. In a conventional system, electron beam focusing involves adjusting the beam optical system with its quadrupole coils, main solenoid coil and trim solenoid coil. Electrical current through each of these coils must be separately controlled and varied as a function of time to achieve proper beam focus. Controlling all of the scanner optics, including two dipole coils, requires five separate time varying currents and one fixed current.
Although the quadrupole magnets provide a flexible mechanism to control the beam profile, this flexibility can greatly complicate tuning the electron beam. Unacceptable beam profiles that can damage the target may be generated. For example, the beam profile ellipse at the target may tilt out of the radial plane, or may assume an unacceptable size in the radial plane. U.S. Pat. No. 4,631,741 discloses the use of "W-wire" type monitors 56 (see FIG. 2) installed on a target ring to provide data useful in avoiding unacceptable and potentially dangerous beam profiles during beam tuning.
Thus, there is a need for a method and apparatus that focuses an electron beam spot on a target, without requiring quadrupole and solenoid coils, "W-wire" monitors, and the attendant time consuming adjustment. Preferably such method and apparatus should still provide flexibility in focusing and tuning the electron beam, and should prevent unacceptable beam profiles that may damage the target.
The present invention provides such a method and an apparatus.